Low consumption assisted flare apparatus and method

ABSTRACT

An assisted flare apparatus and method which increase the amount of air which is inducted into each of the flare combustion tip assemblies used in the apparatus by injecting the steam or other assist media used in each tip assembly into an inner conduit of the tip assembly which is configured such that at least a lower portion of the inner conduit is in the form of a venturi flow structure in which a straight venturi flow section is followed by a diverging venturi bell outlet section.

FIELD OF THE INVENTION

The present invention relates to flare apparatuses and methods in which steam or other entrainment media is injected to (i) induce additional air into the flare combustion zone, (ii) create turbulence which enhances mixing and improves combustion efficiency, (iii) inhibit smoke generation, and (iv) provide reduced emissions.

BACKGROUND OF THE INVENTION

Flare systems are used in many applications to burn off fuels, waste gases, and other gases that can be harmful if vented to the atmosphere without burning. Flare systems are widely used in the refining, chemical, petrochemical, petroleum production, and other industries for burning flammable and/or toxic materials which are released due to upset or startup conditions, or which are released simply as a result of the process itself.

Most utility-type flares are comprised of a pipe with a flame stabilization device at the top thereof. Flare gas flows through the pipe and burns at the stabilization device. However, when the flow of flare gas becomes too great for the amount of air that will naturally mix with the flare gas at the combustion zone, the combustion of the flare gas occurs under sub-stoichiometric conditions. When the flare gas is burned under sub-stoichiometric conditions, smoke will typically be produced. Smoke produced by flare systems contributes to smog. In addition, the combustion of the flare gas under sub-stoichiometric conditions produces atmospheric emissions such as: nitrogen oxides (NO_(x)); carbon monoxide (CO); unburned hydrocarbon emissions (UHC's); and particulates (PM10).

To reduce smoke production, as well as the production of harmful emissions, steam-assisted flare assemblies have been used in which steam is injected (i) into the flare gas stream flowing upwardly through the flare stack, (ii) around the upper end of the stack, and/or (iii) into one or more tubes within the flare stack. Depending upon the type and location of the injection system, the injected steam can operate to: induce additional air into the combustion mixture; create increased turbulence for better mixing; lower the combustion flame temperature for reduced NO_(x) emissions and less thermal cracking; shape the combustion flame; cool the flare tip; and/or reduce noise.

However, although the benefits provided by steam-assisted flare systems can be significant, the fuel costs and other costs incurred for producing and delivering the amount of high pressure steam which these systems require can also be substantial. In addition, due to the offsetting emissions which are produced as a result of the combustion of gas or liquid fuels for generating the steam used in the steam-assisted flare systems, the benefits provided by these systems for the flare combustion process are somewhat nullified.

Consequently, a need exists for an improved assisted flare apparatus and method in which the amount of steam or other entrainment media required for inducing a desired amount of air into the flare combustion mixture to provide a smokeless operation is significantly reduced.

SUMMARY OF THE INVENTION

The present invention satisfies the needs and alleviates the problems discussed above. In the inventive apparatus and method, venturi structures are used in one or more flare tip nozzle assemblies to increase the amount of air which is inducted into the flare combustion zone. The inventive apparatus and method are well suited for use in elevated flares, but can also be used in enclosed flares and other types of flare systems.

In one aspect, there is provided an apparatus for combusting a flare gas comprising one or more flare nozzle assemblies. Each of the flare nozzle assemblies preferably comprises: (i) an outer conduit having a longitudinal axis, a rearward longitudinal end, and a forward longitudinal end and (ii) an inner conduit for a flow of an entrainment media and air. The inner conduit preferably comprises: an inlet opening at a rearward longitudinal end of the inner conduit; a discharge opening at a forward longitudinal end of the inner conduit; a longitudinally extending straight venturi section having a substantially constant interior cross-sectional area; and a longitudinally extending diverging venturi section which increases in interior cross-sectional area as the diverging venturi section extends forwardly from a forward longitudinal end of the straight venturi section. Each flare nozzle assembly also preferably comprises an injection structure positioned outside of the inlet opening of the inner conduit for injecting the entrainment media into the inlet opening of the inner conduit.

The inner conduit of each nozzle assembly preferably extends into the rearward longitudinal end of the outer conduit such that (i) the rearward longitudinal end of the inner conduit is positioned outside of the rearward longitudinal end of the outer conduit and (ii) the discharge opening of the inner conduit is positioned substantially at the forward longitudinal end of the outer conduit. In addition, a longitudinally extending flow annulus for the flare gas is formed in the outer conduit between the interior surface of the outer conduit the exterior of the inner conduit. The longitudinally extending flow annulus (i) laterally surrounds the longitudinal axis of the outer conduit, (ii) has an inlet opening at the rearward longitudinal end of the outer conduit for receiving the flare gas, and (iii) has a discharge opening for the flare gas at the forward longitudinal end of the outer conduit.

In another aspect, there is provided a method of combusting a flare gas which preferably comprises the steps of: (a) delivering the flare gas upwardly through an annulus between an inner surface of an outer conduit and an exterior of an inner conduit, the inner conduit extending upwardly into a lower end of the outer conduit, the annulus having a discharge opening at an upper end of the outer conduit, the inner conduit having a discharge opening substantially at the upper end of the outer conduit, and the inner conduit having an inlet opening at a lower end of the inner conduit which is positioned outside of and spaced downwardly from the lower end of the outer conduit; (b) injecting an entrainment media into the inlet opening of the inner conduit from an injection structure which is spaced downwardly from the inlet opening of the inner conduit so that the entrainment media inducts an amount of air into the inner conduit as the entrainment media travels through a gap between the injection structure and the inlet opening of the inner conduit; and (c) increasing the amount of air inducted into the inner conduit by the entrainment media in step (b) by delivering the entrainment media upwardly through a venturi structure, which forms at least a lower portion of the inner conduit, and out of the discharge opening of the inner conduit substantially at the upper end of the outer conduit. The venturi structure comprises (i) an upwardly extending straight venturi section of the inner conduit having a substantially constant interior cross-sectional area followed by (ii) a diverging venturi section of the inner conduit which increases in interior cross-sectional area as it extends upwardly from the straight venturi section.

Further aspects, features, and advantages of the present invention will be apparent to those in the art upon examining the accompanying drawings and upon reading the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational side view illustrating an embodiment 2 of the flare apparatus provided by the present invention.

FIG. 2 is an elevational side view of a portion of the inventive flare apparatus 2 illustrating a flare tip nozzle assembly 8 used in the inventive flare apparatus 2.

FIG. 3 is an elevational view of an inner conduit 15 used in the inventive flare apparatus 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment 2 of the flare apparatus provided by the present invention is illustrated in FIGS. 1-3. The inventive flare apparatus 2 comprises a plenum 4 on the upper end of a flare stack 6. A flare gas is delivered to the plenum 4 through the flare stack 6. The inventive flare apparatus 2 further comprises one or more (preferably a plurality) of flare tip nozzle assemblies 8 which receive the flare gas from the plenum 4 and discharge the flare gas from the upper (or forward) ends 10 of the nozzle assemblies 8 for combustion. As used herein and in the claims, the term “flare gas” can refer to any type of single or two phase fuel, waste gas, or other stream from a plant, refinery, oil or gas production site, or other source which is suitable for combustion in a flare system.

Each of the one or more flare tip nozzle assemblies 8 of the inventive apparatus 2 preferably comprises: an outer conduit 14; an inner conduit 15; a flow annulus 16 for the flare gas which is formed between the inner surface 18 of the outer conduit 14 and the exterior surface 20 of the inner conduit 15; an injection structure 22 for injecting steam or other entrainment media into the lower (or rearward) longitudinal end 26 of the inner conduit 15; and an exterior clean-up ring or assembly 28 positioned around the upper (or forward) longitudinal end 30 of the outer conduit 14.

The outer conduit 14 of each nozzle assembly 2 comprises; a longitudinal axis 32; a longitudinally extending outer wall 34 which surrounds the longitudinal axis 32; an inlet opening 36 at the lower (or rearward) longitudinal end 38 of the outer conduit 14; and a discharge opening 40 at the upper (or forward) longitudinal end 30 of the outer conduit 14. The flow annulus 16 formed between the inner surface 18 of the outer conduit 14 and the exterior 20 of the inner conduit 15 also surrounds the longitudinal axis 32 of the outer conduit and extends longitudinally from the lower longitudinal end 38 to the upper longitudinal end 30 of the outer conduit 14. The flow annulus 16 has an inlet opening 42 at the lower longitudinal end 38 of the outer conduit 14 and a discharge opening 44 at the upper longitudinal end 30 of the outer conduit 14.

The outer conduit 14 is preferably cylindrical but can alternatively have an elliptical, oval, rectangular, square, triangular or other cross-sectional shape. In addition, although the interior of the outer conduit 14 will preferably have a substantial constant interior cross-sectional area along its entire length, it will be understood that the entire length or any portion of the longitudinal length of the interior surface 18 of the outer conduit 14 can alternatively have a converging or diverging shape.

The outer conduit 14 of each nozzle assembly 8 is connected to the plenum 4 in a manner which places the inlet opening 42 of the nozzle flow annulus 16 in fluid communication with the interior 46 of the plenum 4. In the embodiment 2 of the inventive flare apparatus illustrated in FIGS. 1 and 2, the lower end 38 of the outer conduit 14 is connected to the upper wall 48 of the plenum 4 such that the inlet opening 42 of the flow annulus 16 is positioned over a flow opening 50 formed through the upper wall 48. Alternatively, a lower portion of the outer conduit 14 can extend through an opening in the upper wall 48 of the plenum 4 such that the lower end 38 of the outer conduit 14 and the inlet opening 42 of the flow annulus 16 would be located inside the interior 46 of the plenum 4.

The upper wall 48 of the plenum 4 can be flat, outwardly dished, or have any other shape.

The inner conduit 15 of each nozzle assembly 8 extends upwardly (or forwardly) through the lower wall 51 of the plenum 4 and into the lower end 38 of the outer conduit 14 such that (a) the upper (or forward) longitudinal end 52 of the inner conduit 14 is located substantially at (i.e., preferably within ±40 inches of, more preferably within ±30 inches of, more preferably within ±20 inches of, more preferably within 10 inches of, more preferably within ±5 inches of, more preferably within ±2.5 inches of, more preferably within ±+1 inch of, and most preferably at) the upper longitudinal end 30 of the outer conduit 14, (b) the lower (or rearward) longitudinal end 26 of the inner conduit 15 is located outside of and spaced downwardly (or rearwardly) from the lower longitudinal end 38 of the outer conduit 14, and (c) the inlet opening 56 at the lower end 26 of the inner conduit 15 is located outside of the plenum 4. The lower wall 51 of the plenum can be flat, outwardly dished, or have any other desired shape.

The inner conduit 15 of each nozzle assembly 8 is configured such that at least a lower portion of the inner conduit 15 is in the form of a venturi flow structure 58 for the steam or other entrainment media which is injected into the lower end 26 of the inner conduit 15. The flow of the steam or other entrainment media through the venturi flow structure 58 of the inner conduit 15 operates to further reduce the pressure at the lower inlet opening 56 of the inner conduit 15 so that, as compared to an entirely straight inner conduit design with no venturi flow, significantly more air is inducted by the entrainment media into the inner conduit 15.

The venturi flow structure 58 of the inner conduit 15 preferably comprises: a venturi inlet bell structure 60 at the lower (or rearward) end 26 of the inner conduit 15; a straight venturi section 62 which extends upwardly (or forwardly) from the inlet bell structure 60; and a diverging venturi bell outlet section 64 which extends upwardly (or forwardly) from the upper or forward end of the straight section 62. The interior cross-sectional flow area of the straight venturi section 62 is substantial constant along the entire length of the straight section 62. The interior cross-sectional flow area of the diverging bell outlet section 64 increases as the bell outlet section 64 extends upwardly or forwardly from the straight venturi section 62. The interior surface of the bell outlet section 64 preferably diverges at an angle 66 in the range of from about 5° to about 60°, more preferably from about 5° to about 45° and more preferably from about 5′ to about 30° with respect to the longitudinal axis 32 of the outer and inner conduits 14 and 15.

The diverging bell outlet section 64 of the venturi structure 58 can extend all of the way to the discharge opening 68 at the upper (or forward) longitudinal end 52 of the inner conduit 15. Alternatively, the inner conduit 15 can further comprise an upper or forward discharge section 69 of substantially constant internal cross-sectional flow area which extends from the upper or forward end 65 of the diverging venturi bell outlet section 64 to the discharge opening 68 of the inner conduit 15. If the inner conduit 15 includes an upper or forward discharge section 69 as just described, the upper or forward end 65 of the diverging bell outlet section 64 of the venturi structure 58 can be located prior to or inside the outer conduit 14.

The longitudinal length of the venturi flow structure 58 will preferably be at least 10%, more preferably at least 20% and more preferably at least 25%, of the entire longitudinal length of the inner conduit 15. The longitudinal length of the straight venturi section 62 of the venturi flow structure 58 will preferably be at least 5%, more preferably at least 10%, of the entire length of the venturi structure 58. The longitudinal length of the diverging bell outlet section 64 of the venturi structure 58 will preferably be at least 10%, more preferably at least 20% and more preferably at least 30%, of the entire length of the venturi structure 58.

The inner conduit 15 and each section of the venturi flow structure 58 of the inner conduit 15 will preferably have a circular cross-sectional shape such that, at least in the interior thereof, the interior surface of the straight venturi section 62 is cylindrical and the interior surface of the diverging venturi bell section 64 has a conical shape. However, the cross-section of the inner conduit 15 and of each section of the venturi structure 58 can alternatively be elliptical, oval, rectangular, square, triangular or other non-circular shape. Also, the interior surface 59 of the venturi flow structure 58 can be smooth or roughened.

The entrainment media used in the inventive apparatus and method will preferably be steam. However, examples of other types of entrainment media suitable for use in the inventive apparatus and method include, but are not limited to, carbon dioxide, air, nitrogen, or argon.

The injection structure 22 for the entrainment media is located outside of the plenum 4 and is spaced downwardly or rearwardly apart from the inlet bell opening 56 at the lower end 26 of the inner conduit 15 so that a gap 70 is formed between the injection structure 22 and the inlet bell opening 56 of the inner conduit 15. The injection structures 22 for the entrainment media can be single port tips, multi-port spiders, or other types of ejectors for supersonic, sonic, or subsonic ejection flow.

In the method of the present invention, the flare gas delivered to the plenum 4 flows upwardly or forwardly through the flow annulus 16 of each flare tip nozzle assembly 8 and is discharged for combustion from the discharge opening 44 of the flow annulus 16 at the upper or forward longitudinal end 30 of the outer conduit 14. At the same time, steam or other entrainment media is ejected from the injection structure 22 toward the inlet bell opening 56 at the lower or rearward end 26 of the inner conduit 15. As the ejected entrainment media travels through the gap 70 between the injection structure 22 and the inlet bell opening 56, atmospheric air is entrained by the media and pulled into the inner conduit 15.

In accordance with the present invention, by causing the injected entrainment media to flow through the venturi flow structure 58 which forms at least a lower or rearward portion of the inner conduit 15, the pressure at the inlet bell opening 56 is further reduced, thus resulting in significantly more air being induced by the flow of the entrainment media into the inner conduit 15. As compared to a nozzle tip using an entirely straight inner conduit design, the venturi flow structure 58 incorporated in the inner conduit 15 of the inventive flare tip nozzle assembly 8 increases the amount of air delivered by the entrainment media into the inner conduit 15 by an amount of at least 5% (more preferably at least 10%, at least 15%, at least 20%, or at least 25%).

The air inducted into the inner conduit 15 mixes with the entrainment media and is discharged from the discharge opening 68 of the inner conduit 15 for combustion of the inducted air with the flare gas discharged from the discharge opening 44 of the flow annulus 16. Because the discharge opening 68 of the inner conduit 15 is located substantially at the upper or forward longitudinal end 30 of the outer conduit 14, rather than inside the outer conduit 14, substantially no mixing of the flare gas with the entrainment media and inducted air occurs within the outer conduit 14, thus further reducing the pressure at the inlet opening 56 of the inner conduit 15 and causing even more air to be inducted into the inner conduit 15.

The exterior clean-up ring or apparatus 28 at the upper (or forward) longitudinal end 30 of the outer conduit 14 comprises a series of ejection ports, nozzles or other ejection structures which eject steam into the combustion mixture discharged from the flare tip nozzle assemble 8. The steam ejected from the clean-up ring 28 operates to induct additional air into the combustion mixture and can also be used to shape the combustion flame or assist in maintaining the shape of the flame in high wind conditions.

Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those in the art. Such changes and modifications are encompassed within the invention as defined by the claims. 

What is claimed is:
 1. An apparatus for combusting a flare gas comprising one or more flare nozzle assemblies, each of the flare nozzle assemblies comprising: an outer conduit having a longitudinal axis, a rearward longitudinal end, and a forward longitudinal end; an inner conduit for a flow of an entrainment media and air, the inner conduit comprising an inlet opening at a rearward longitudinal end of the inner conduit, a discharge opening at a forward longitudinal end of the inner conduit, a longitudinally extending straight venturi section of substantially constant interior cross-sectional area, and a longitudinally extending diverging venturi section which increases in interior cross-sectional area as the diverging venturi section extends forwardly from a forward longitudinal end of the straight venturi section; an injection structure positioned outside of the inlet opening of the inner conduit for injecting the entrainment media into the inlet opening of the inner conduit; the inner conduit extending into the rearward longitudinal end of the outer conduit such that (i) the rearward longitudinal end of the inner conduit is positioned outside of the rearward longitudinal end of the outer conduit and (ii) the discharge opening of the inner conduit is positioned substantially at the forward longitudinal end of the outer conduit; and a longitudinally extending flow annulus for the flare gas which is formed in the outer conduit between an interior surface of the outer conduit an exterior of the inner conduit, wherein the longitudinally extending flow annulus (i) laterally surrounds the longitudinal axis of the outer conduit and (ii) has an inlet opening at the rearward longitudinal end of the outer conduit for receiving the flare gas.
 2. The apparatus of claim 1 wherein: the inner conduit further comprises a venturi inlet bell structure at the rearward longitudinal end of the inner conduit and the longitudinally extending straight venturi section of the inner conduit extends forwardly from the venturi inlet bell structure.
 3. The apparatus of claim 1 wherein the inner conduit further comprises a longitudinal discharge section of substantially constant internal cross-sectional area which extends forwardly from a forward longitudinal end of the longitudinally extending diverging venturi section of the inner conduit to the discharge opening of the inner conduit.
 4. The apparatus of claim 3 wherein the forward longitudinal end of the longitudinally extending diverging venturi section of the inner conduit is located outside of the rearward longitudinal end of the outer conduit.
 5. The apparatus of claim 3 wherein the forward longitudinal end of the longitudinally extending diverging venturi section of the inner conduit is located inside of the outer conduit.
 6. The apparatus of claim 1 further comprising a plenum for the flare gas, the plenum having an interior, an upper wall, and a lower wall and, wherein for each of the one or more flare nozzle assemblies: the rearward longitudinal end of the outer conduit of the flare nozzle assembly is located inside the interior of the plenum or at the upper wall of the plenum such that the inlet opening of the flow annulus of the flare nozzle assembly is in fluid communication with the interior of the plenum; the forward longitudinal end of the outer conduit of the flare nozzle assembly is located outside of plenum; the inner conduit of the flare nozzle assembly extends through the lower wall of the plenum; and the inlet opening of the inner conduit of the flare nozzle assembly is located outside of the plenum.
 7. The apparatus of claim 6 comprising a plurality of the flare nozzle assemblies.
 8. The apparatus of claim 6 further comprising a flare stack having an upper end on which the plenum is located.
 9. The apparatus of claim 1 wherein the longitudinally extending straight venturi section of the inner conduit has a cylindrical interior surface and the longitudinally extending diverging venturi section of the inner conduit comprises an interior surface having a conical shape.
 10. The apparatus of claim 1 further comprising an upper steam injection ring or assembly positioned outside of and around the forward longitudinal end of the outer conduit.
 11. A method of combusting a flare gas comprising the steps of: a) delivering the flare gas upwardly through an annulus between an inner surface of an outer conduit and an exterior of an inner conduit, the inner conduit extending upwardly into a lower end of the outer conduit, the inner conduit having a discharge opening substantially at the upper end of the outer conduit, and the inner conduit having an inlet opening at a lower end of the inner conduit which is positioned outside of and spaced downwardly from the lower end of the outer conduit; b) injecting an entrainment media into the inlet opening of the inner conduit from an injection structure which is spaced downwardly from the inlet opening of the inner conduit so that the entrainment media inducts an amount of air into the inner conduit as the entrainment media travels through a gap between the injection structure and the inlet opening of the inner conduit; and c) increasing the amount of air inducted into the inner conduit by the entrainment media in step (b) by delivering the entrainment media upwardly through a venturi structure, which forms at least a lower portion of the inner conduit, and out of the discharge opening of the inner conduit substantially at the upper end of the outer conduit, wherein the venturi structure comprises (i) an upwardly extending straight venturi section of the inner conduit having a substantially constant interior cross-sectional area followed by (ii) a diverging venturi section of the inner conduit which increases in interior cross-sectional area as it extends upwardly from the straight venturi section.
 12. The method of claim 11 wherein the entrainment media is steam.
 13. The method of claim 11 wherein the inner conduit further comprises a venturi inlet bell structure at the lower end of the inner conduit.
 14. The method of claim 11 wherein the inner conduit further comprises an upper discharge section having a substantially constant internal cross-sectional area which extends upwardly from the diverging venturi section of the inner conduit to the discharge opening of the inner conduit. 